For producing a packaging material for foods or medical or cosmetic articles, a packaging material for industrial parts, or the like, a technique has been employed in which an olefin resin such as an ethylene resin or propylene resin is laminated to a base such as, e.g., any of various resin films or sheets, any of various metal foils or sheets, or paper through an adhesive resin layer to produce a laminate and thereby impart properties such as thermal fusion bondability, water vapor barrier properties, water proofness, and rust preventive properties to the base.
A packaging material effective in inhibiting the contents from oxidatively deteriorating in quality is being provided which comprises a polyolefin resin layer excellent in formability, rigidity, and resistance to the contents and an ethylene/vinyl alcohol copolymer resin (EVOH) layer formed over the polyolefin resin layer through an adhesive resin layer to impart oxygen barrier properties to the polyolefin resin layer.
Examples of laminating methods for producing such packaging materials include glue laminating in which a base is bonded to an olefin resin film or sheet with an adhesive to produce a laminate and extrusion laminating in which an olefin resin is extrusion-laminated to a base through an anchor coat to produce a laminate.
In the case of producing the laminated film or sheet comprising a polyolefin resin layer and an EVOH layer superposed thereon through an adhesive resin layer, coextrusion molding is frequently used.
Adhesive resin compositions for use in such laminates, laminated sheets, or laminated films are required to have high-temperature adhesiveness as well as initial adhesiveness and long-term adhesiveness. Namely, in applications such as, e.g., food packaging materials to be subjected to a high-temperature treatment such as retortion or boiling sterilization or industrial materials to be used at high temperatures, heat-resistant adhesiveness is important. There are cases where the adhesive resin compositions are further required to have transparency for ascertaining the contents and mechanical strength.
JP-A-2002-194174 proposes the following composition as such an adhesive resin composition.
“An adhesive resin composition characterized by comprising the following resin ingredient (A) and the following resin ingredient (B) in amounts of 10-99.5% by weight and 0.5-30% by weight, respectively, based on all resin ingredients (100% by weight):
resin ingredient (A): a propylene polymer comprising the following component (a1) and component (a2) and obtained by yielding component (a1) by polymerizing and then producing component (a2) by polymerization,
component (a1): a propylene homopolymer having an isotactic index of 90% or higher or a copolymer of propylene and another α-olefin having 2-8 carbon atoms, the amount of component (a1) being 10-60% by weight based on the whole resin ingredient (A),
component (a2): a copolymer of propylene as an essential component and one or more other α-olefins having 2-8 carbon atoms including ethylene as another essential component, wherein the content of room-temperature-xylene insolubles in the copolymer is higher than 20% by weight and not higher than 70% by weight based on the whole resin ingredient (A), the content of room-temperature-xylene solubles in the copolymer is 10-60% by weight based on the whole resin ingredient (A), and the room-temperature-xylene solubles have a content of α-olefins excluding propylene lower than 20% by weight, the amount of component (a2) being 40-90% by weight based on the whole resin ingredient (A); and
resin ingredient (B): polypropylene modified with an unsaturated carboxylic acid or anhydride thereof so that the content of the unsaturated carboxylic acid or anhydride thereof is 0.01% by weight or higher.”
Incidentally, the resin ingredient (A) contained in that adhesive resin composition is one obtained by successive polymerization in which component (a1) is yielded by polymerization and component (a2) is then yielded by polymerization. This resin ingredient (A), which is obtained by such successive polymerization, has a structure comprising a sea phase constituted of component (a1) and an island phase constituted of component (a2) evenly dispersed as fine particles in the sea phase.
A polyolefin composition obtained by such successive polymerization is disclosed in JP-A-6-25367. However, JP-A-6-25367 includes no suggestion about any formulation for use as an adhesive resin composition.
Patent Document 1: JP-A-2002-194174
Patent Document 2: JP-A-6-25367
Patent Document 3: JP-A-2001-294837